1. Technical Field
The present invention relates to a semiconductor memory using an electrically erasable nonvolatile memory which is called a flash EEPROM or flash memory. In particular, to a semiconductor external storage system which can replace the magnetic disk in a conventional computer system.
2. Background Art
As portable personal computers such as those commonly knows as notebook computers have spread, the requirement for small-sized, lightweight and low power consumption computer systems has become increasing strong. The external storage system using a semiconductor memory has a low power consumption and can operate at a high speed because it does not use a mechanical drive system such as the magnetic disk, in addition, it consists of small memory modules, and thus it is small-sized, lightweight, and has a large degree of freedom with respect to shape as compared with the magnetic disk, and is also easily made in the form of a card.
However, the conventional semiconductor memory has many problems with respect to such points as capacity and battery backup. If SRAM is used as the memory, the cost is high and the capacity becomes small though the backup time by a battery becomes long. For DRAM which is excellent in cost and capacity, the standby power consumption is large and the backup time is limited to one week or so. There is also a danger of data loss due to a problem in the battery system. EEPROM is costly though it requires no battery.
A block erase type flash memory has been developed as a memory to solve these problems. Its memory element consists of one transistor as DRAM, and it can be provided with high density and it is expected to have a bit cost equivalent to or less than DRAM (low cost, large capacity) depending on the future market. The memory element is nonvolatile and requires no battery backup. The outline of such flash memory is introduced by Richard D. Pashley et al. in "Flash memories: the best of two worlds", IEEE SPECTRUM, December 1989, pp. 30-33. Also, a similar flash memory is disclosed in Published Unexamined Patent Application No. 2-10598.
However, the flash memory has limitations which SRAM and DRAM do not have. First, the erase/program cycles have an upper limit of the order of 100,000. In addition, the programming of memory bits is a one-way process and change is allowed only from 0 to 1 or 1 to 0. For change in the opposite direction, it is necessary to set the whole memory block to 0 or 1 by block erasure. Erasing usually takes several tens of milliseconds, and block erasure requires special procedures such as verify which takes a further several seconds.
If a semiconductor memory consisting of such flash memory is connected to the bus of a host computer as an alternate to the traditional magnetic disk, a specific sector is very often written by the host computer and reaches the upper limit of the erase/program cycles considerably earlier than other sectors. Further, the sector writing takes a lot of time. The reason for this is that, to modify several bytes in the sector, all the data within the memory block including the sector are temporarily saved in the memory space of the host computer, and new data is written back to empty sectors after erasing of the memory block. It takes several seconds to write a sector in a semiconductor memory having a capacity of 1M bits or more. In addition, a special program is required to connect a semiconductor memory to the bus of the host computer.
To solve these problems, the development of a sector erase type flash memory is needed. For instance, 27F010 from SEEQ TECHNOLOGY CORPORATION (1024 K flash EPROM) allows not only the chip erasure which erases all the bits of the memory chip (change to logical one) but also the sector erasure which erases only a specific sector. In this scheme, the saving or writing back of sectors other than those to be erased can be avoided. However, it takes on the order of several tens of milliseconds because the erasing of old sectors is done along with the writing of sectors, and thus the obtained performance is equivalent to or less than the magnetic disk. In addition, the problem is not solved that a particular sector is written very often by the host computer and reaches the allowed erase/program cycles somewhat earlier than other sectors, and thus the sectors cannot effectively be used as the whole flash memory. Further, the sector erase type has a chip structure which is more complicated than the block erase type, and it is said that there are also problems with respect to cost and erase/program cycles.
Incidentally, many of these problems are considered to be due to the control scheme in which the block and sector adresses of the external storage are controlled by the host computer, that is, the physical address of the external storage is determined by the logical address possessed by the command of the host computer.